I have never used one, saving up for one tho! I think it would be great! We have tried so many different positions using pillows and such, different angles are great and I can think of sooo many ways we could explore the potential uses!
No experience with the lesbian side of the question - but I can tell you we love our wedge. Raising her hips off the bed helps with oral, missionary and doggie style sex. For penetrative sex (with a dildo or the real thing) the angle of the wedge is designed to maximize g-spot stimulation - and it does.
And as chpsyglot says - it makes oral much more comfortable - especially if you add any anal play to the equation.